The UK would lose its top credit score if the public voted to leave the EU in June’s referendum, ratings agency Standard & Poor’s said in a fresh warning on Thursday.

The only big credit ratings agency to award the country the highest AAA ranking renewed its warning over the UK’s economic prospects outside the EU after the prime minister David Cameron fired the starting gun on referendum campaigning last weekend.

A senior director at the ratings agency, which assigns credit scores to debt issued by governments around the world, said a Brexit vote “certainly could” affect Britain’s rating, when interviewed by Bloomberg Television. Such a downgrade would push up the UK’s government borrowing costs and hurt its standing in international markets.

Moritz Kraemer, chief ratings officer for S&P, echoed warnings made earlier this week from rival agencies Fitch and Moody’s over the UK’s credit rating in the event of a leave vote.

“We’ve been quite unambiguous saying that if Britain leaves the EU, the sovereign rating would go down by at least a notch,” Kraemer said.

“Remember we still rate the UK AAA with a negative outlook. We think that Britain has been benefiting from EU membership; Britain has been one of the magnets for inbound investment, not only from the rest of the EU but globally. It’s our belief that a big part of this attraction of Britain, among other things, but it’s also that it’s a member of the EU and it’s therefore an integral part of the largest trading area in the world.”

Asked on Thursday about the size of a potential downgrade, Kraemer said it “really depends on the circumstances”.

“It depends on what would come next, because the easy part would be to vote to leave, but the difficult part would be then to negotiate something else that would come in its place,” he told Bloomberg.

Such a renegotiation process could take several years given any deal would need to be approved by all EU members, said Kraemer. “Depending on what the political realities look like, and with a lot of tension in the EU even today, you know, the outcome might actually be worse than a one notch downgrade would suggest.”

On Monday, Moody’s put the government on alert that a decision to leave the EU could lead to a downgrade of the UK’s strong credit score. It said the outcome of the referendum “remains too close to call” but that in the event of a vote to leave the EU, the economic costs would outweigh the benefits. The UK is rated Aa1 by Moody’s, one notch below the top triple-A score.

Fitch, which cut its rating on the UK to a notch below the top AAA level in 2013, said on Monday that leaving the EU would bring short-term disruption and long-term risks for the UK.